1. Field
This disclosure relates generally to adaptive predistortion and, more specifically, to techniques for adaptive predistortion direct current offset correction in a transmitter.
2. Related Art
In general, due to technology limitations, power amplifiers (PAs) are inherently non-linear devices that may distort a signal to be transmitted causing adjacent channel leakage and out-of-band emissions. Moreover, distortion attributable to PA non-linearity may hinder symbol recovery of a transmitted signal at a receiver. Adaptive predistortion (APD) is a known approach that has been utilized to address transmitter PA non-linearity. According to the APD approach, a function that represents an inverse of a PA transfer function is multiplied with a baseband input signal in order to linearize an overall PA gain at an output of the PA. Typically, predistortion is implemented in an adaptive closed-loop configuration in which a PA output signal is fed back to an APD unit that compares the PA output signal (or a portion thereof) with a baseband input signal to facilitate minimization of distortion in transmitted signals. The feedback arrangement allows the predistortion function to be adapted to changes in PA characteristics due to temperature, etc.
In a typical APD configuration, an adaptive algorithm is employed to compare a delayed baseband input signal with a fed back PA output signal to provide an error signal that drives a look-up table. Based on the error signal, an entry (e.g., a transfer function) from the look-up table is selected to provide a predistortion function that is multiplied with the baseband input signal. Ideally, the error signal is zero when the closed-loop configuration settles. Unfortunately, even a small direct current (DC) offset (after coarse DC offset correction) may cause a transmitter to exhibit excessive spurious emissions. For example, a few millivolts DC offset in an APD feedback path of a transmitter may result in excessive out-of-band leakage (e.g., modulation output radio frequency spectrum (mod ORFS) figures) that may cause the transmitter to fail specifications.
Traditionally, one of three approaches have usually been employed in digital (and mixed) transceiver integrated circuits (ICs) for direct current (DC) offset correction. A first DC offset correction approach has implemented an accumulator and dump technique that has averaged samples in the absence of a modulated signal. Unfortunately, a DC offset may vary when a signal is modulated. A second DC offset correction approach has implemented an in-line low-pass filter technique that usually works well in very low intermediate frequency (VLIF) transceiver architectures. Unfortunately, the in-line low-pass filter technique inserts extra delay in a transmit path. A third DC offset correction approach has implemented a feedback loop that employs one or more comparators. While relatively accurate, implementing comparators in a feedback loop increases an analog area of an associated IC.